Integrated circuit design involves assembling the combination of circuits that perform the desired function. It is typical to construct the desired circuits from building blocks called cells. Each cell performs a specific function and has a defined set of inputs and outputs. The set of such cells available to the designer is called the library of cells. A selected mix of such cells are assembled depending upon the desired function of the integrated circuit.
Integrated circuit design generally uses a variety of cells which perform the same function. These differing cells are employed in different contexts with differing needs.
A crucial factor in the performance of an integrated circuit is the cell timing. Complex circuits often use the cells in cascade with one or more cell outputs driving another cell input. Integrated circuits are typically clock rate driven. This means that many such serial chains of cells must provide their outputs before the expiration of an interval of time. Proper circuit function depends upon meeting such timing restraints in virtually all of the integrated circuit.
The delay of a cell, that is the length of time from receiving signals at its inputs to generating signals at its outputs is primarily a function of the rail-to-rail supply voltage, the input slew and the output load capacitance. The supply voltage is typically the same for the whole integrated circuit or the integrated circuit is divided into a small number of power domains with differing supply voltages. Thus the metric that has a cascading effect on timing here is the output slew of a cell.
An increasing problem with digital circuits is leakage current. Leakage current is the current a circuit draws when nominally turned OFF. With increased circuit density made possible by smaller circuit features, more circuits can be constructed on a single integrated circuit. In addition, these same smaller features cause increased leakage current for nominally the same circuit types embodied in smaller features. As a consequence, leakage current is a major portion of the total current drawn by state of the art integrated circuits. This increasingly important leakage current causes problems particularly for integrated circuits intended for portable, battery-powered use.